Databases Reference
In-Depth Information
After the system has been running for a day or two with the DB_CACHE_
ADVICE parameter turned on, Janice reviews the dynamic performance view
V$DB_CACHE_ADVICE:
select size_for_estimate, estd_physical_reads
from v$db_cache_advice;
SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READS
----------------- -------------------
4 1158418
8 213691
12 100625
16 44844
20 37598
24 35000
28 34727
32 34590
36 34590
40 34590
44 34590
48 34590
52 34590
56 34590
60 34590
64 34590
68 34590
72 34590
76 34590
80 34590
20 rows selected.
The first column, SIZE_FOR_ESTIMATE, is the proposed size for the buffer
pool in megabytes. The second column, ESTD_PHYSICAL_READS, is the number
of reads from disk that would occur with the corresponding buffer cache size,
given the recent activity level. From this report, Janice sees that her buffer cache
of 25MB is sized optimally. Increasing the buffer cache size to 28MB, for exam-
ple, would reduce the physical I/O only slightly, and it probably would not jus-
tify a memory upgrade at this time. At 32MB and higher, the additional memory
allocated to the buffer cache would not reduce the reads from disk at all. It
appears that Janice will not need a memory upgrade on the server for the fore-
seeable future.
